Optical scattering techniques are in wide use for inspecting highly polished surfaces, such as those of lenses and silicon wafers as employed as starting materials in semiconductor manufacture. These techniques involve directing a beam of light, typically a focused beam of coherent light from a laser, onto the surface. Most of the beam is "specularly" reflected, that is, is reflected at an angle of reflection equal to the angle of incidence, as from a mirror; however, a small fraction of the beam is "scattered" into other directions. The amount of light scattered is generally representative of the roughness of the surface and the presence of particulates thereon or defects therein, as explained further below.
Optical scattering techniques provide a powerful tool for process monitoring in manufacturing environments because of their noncontact nature and relative ease of use; for example, optical scattering techniques are often employed to detect particulate contamination of silicon wafers on fabrication lines. The requirement that particles smaller than the minimum dimension of the features to be fabricated on the wafer can be reliably detected places strict demands on the sensitivity of an instrument to those particles. One important issue that limits the sensitivity of such an instrument to particulates is scattering from the residual substrate microroughness.
The full strength of the optical scattering technique lies in its ability to diagnose deviations from ideal conditions. For example, optical scattering from smooth surfaces, such as mirrors, transparent optics, and silicon wafers, can yield information about the condition of those surfaces. Surface microroughness, particulate contamination, and subsurface defects result from different adverse conditions in the manufacturing environment; distinguishing between these sources of defects can result in improvements in the ability to identify and correct the sources of such conditions.
Current scanning surface inspection systems (often called wafer scanners) employ optical scattering techniques to detect microroughness, particles, and defects in silicon wafers. Light, usually from a laser, is focused onto the surface of the wafer, and optics (in the form of curved mirrors or lenses) collect light that is scattered by the surface and image it onto a sensitive detector, such as a photomultiplier tube. Generally, as the signal from different points on the sample surface is mapped, one observes localized and non-localized scattering. The localized scattering is attributed to particles and defects, and the non-localized signal is attributed to microroughness. The devices illustrated in U.S. Pat. Nos. 4,376,583 and 4,441,124 are representative of such surface inspection systems.
Some degree of microroughness is always present on a surface, and has the tendency to hide detection of the smallest particles. A particle that is smaller than the wavelength of the scattering light beam scatters light in free space with an efficiency proportional to the sixth power of its diameter. Accordingly, the ability to detect small particles is limited by other sources of optical scatter, such as microroughness. Reduction of the microroughness-induced scatter thus improves the detection of these small particles.
In order to lower the proportion of the total scattering signal due to microroughness, it has been generally recognized that such systems should employ p-polarized light incident at an oblique angle with detection of the scattered light out of the plane of incidence. For example, the system shown by U.S. Pat. No. 4,898,471 employs polarized incident light and collects out-of-plane polarized reflected light but employs only a single detection system with sensitivity to a specific polarization, and does not provide a capability for discriminating roughness from particles at other angles.
To increase the solid angle of collection, conventional scattering systems use a large collection optic; however, as the polarization of the scattered light varies with the scattering angle, a single polarization-selective element only nulls the signal at the center of that optic. For example, U.S. Pat. No. 4,668,860 shows the use of polarization discrimination to distinguish bulk from surface scatter but only collects light in the near-specular direction, and by employing a single polarizer on each of the input and output, fails to recognize that the polarization state varies from one solid angle to the next.
U.S. Pat. No. 4,893,932 employs p- and s- polarized light and assumes that each light of both polarizations is scattered but retains its original polarization. This device uses the difference between two scattered signals to determine the nature of the defect. Although the sample is illuminated at an oblique angle, the invention only collects light scattered normal to the surface.
U.S. Pat. No. 5,032,734 employs the rotational dependence of in-plane scattering to acquire information about the orientation of defects in a material, employing polarization of the incident and detected light only to enhance or diminish the transmission of light into and out of the bulk of the material.